The present invention relates to the field of gas turbine technology. It concerns a cooled blade for a gas turbine, where the blade has internal cooling passages located close to the wall of the blade. Cooling fluid such as air flows for convective cooling through the internal cooling passages and is subsequently deflected for external film cooling through film-cooling holes onto the blade surface.
To increase the output and the efficiency, ever increasing turbine inlet temperatures are used in modem gas-turbine plants. In order to protect the turbine blades from the increased hot-gas temperatures, these blades have to be cooled more intensively than was necessary in the past. At high turbine inlet temperatures, both convective cooling and film-cooling elements are used. In order to increase the effectiveness of these types of cooling, it is desirable to reduce the wall-material thicknesses. Furthermore, optimum distribution between convective heat absorption of the cooling fluid and cooling-fluid temperature during the blow-out as a cooling film is desired.
Combinations of convective cooling and film cooling at reduced wall thicknesses have been disclosed, for example, in various publications including WO 99/06672, and patents U.S. Pat. No. 5,562,409, U.S. Pat. No. 4,770,608, and U.S. Pat. No. 5,720,431. In the disclosures, the convective cooling is carried out via impingement cooling, only a small part of the surface being cooled by the respective cooling-fluid jet, which is subsequently used for the film cooling. The convective cooling capacity of the fluid is therefore only partly utilized.
Patents U.S. Pat. No. 5,370,499 and U.S. Pat. No. 5,419,039 describe a method of avoiding this disadvantage. In this case, the cooling fluid is first used for convective cooling in passages close to the wall before it is blown out as a film. At the same time, the convective cooling passages may be provided with turbulence increasing devices (ribs, cylinders or crossed passages). However, the cooling fluid is always directed in these devices in parallel with the main-gas flow, which does not constitute the best solution for optimum cooling.
In the publication WO-Al-99/06672 mentioned above, it has been proposed to direct the cooling fluid in the convective part in an antiparallel manner i.e. in counterflow to the main-gas flow (and thus to the film-cooling flow). This results in cooling which is more homogeneous in the axial direction or in the direction of the hot-gas flow. However, it is still questionable whether homogeneous cooling or temperature distribution in the radial direction is achieved.
In one aspect of the invention, a cooled gas-turbine blade is provided, which also ensures a homogeneous distribution of the material temperature of the blade in the radial direction.
The turbine blade includes a plurality of internal cooling passages and film-cooling holes arranged one above the other in the radial direction of the blade, with the discharge openings of the film-cooling holes being offset from the internal cooling passages, and in particular, the discharge openings of the film-cooling holes lie between the internal cooling passages.
A plurality of internal cooling passages and film-cooling holes are arranged one above the other in the radial direction of the blade in such a way that the discharge openings of the film-cooling holes in each case lie so as to be offset from the internal cooling passages, and in particular lie between the internal cooling passages. Since the cooling effect of the film cooling between the holes is less than in the axial direction downstream of the holes, the cooling effect of the internal cooling is utilized in these intermediate regions by the arrangement according to the invention.
The cooling fluid is first directed in counterflow to the hot-gas flow in convective passages close to the wall, which are integrated in the overall structure and can be provided with turbulence-generating devices that affect the flow of the cooling fluid before the cooling fluid is used for film cooling. As a result, very uniform temperature distributions are produced, which is very important for the small wall thicknesses desired and the low wall thermal resistance associated therewith, since the temperature balance is impaired by heat conduction in the wall at small wall thicknesses. Furthermore, due to the deflection of the cooling fluid, which automatically occurs, an impulse can be applied, and this impulse is advantageous for the cooling effect of the cooling film, as has been described, for example, in Patent U.S. Pat. No. 4,384,823. A swirl can also be produced in the xe2x80x9cprechamberxe2x80x9d of the film-cooling hole, as described in Patent U.S. Pat. No. 4,669,957.
A first preferred embodiment of the blade according to the invention is distinguished by the fact that turbulence-generating elements are arranged in the internal cooling passages. In this way, the contact between cooling fluid and passage wall and thus the internal cooling can be further improved.
Specific amounts of cooling can be achieved if, as in a second preferred embodiment of the invention, cavities are arranged in the internal cooling passages for setting the cooling-fluid pressure or the cooling-fluid mass flow.
The internal cooling can also be improved if, as in another preferred embodiment, first ribs are arranged in the internal cooling passages for enlarging the heat-transfer area. First ribs can be designed so as to alternate in the flow direction as outer ribs and inner ribs, with the inner ribs having a larger height and/or width than the outer ribs.
A further increase in the cooling effect in the interior of the blade is achieved if, as in a further preferred embodiment of the invention, first impingement-cooling holes are provided in order to supply the internal cooling passages. The cooling fluid is passed through the impingement-cooling holes and enters the internal cooling passages in the form of impingement jets.
In addition to the internal cooling passages, a cooling passage may also be arranged in the blade nose. Cooling fluid is admitted into this cooling passage through second impingement-cooling holes. Second film-cooling holes are preferably directed from the cooling passage to the blade surface, the second impingement-cooling holes and the second film-cooling holes are arranged alternately, and second ribs are arranged between the second impingement-cooling holes and the second film-cooling holes for increasing the heat-transfer area and for separating the zones of the cooling passage which belong to the second impingement-cooling holes and the second film-cooling holes.
The internal cooling passages may run axially, and the film-cooling holes may in each case branch off from an associated internal cooling passage at an angle in the radial direction. However, it is also conceivable for the internal cooling passages to run axially, for the ends of the internal cooling passages to be connected by radial passages, and for the film-cooling holes to in each case be arranged between the internal cooling passages and start from the radial passages. Furthermore, it is conceivable in this connection for the internal cooling passages to run at an angle in the radial direction, and for the film-cooling holes to in each case branch off from an associated internal cooling passage in the axial direction. Alternatively, the internal cooling passages can run at a first angle in the radial direction, and the film-cooling holes can in each case branch off from an associated internal cooling passage at a second angle in the radial direction. In all cases, the film-discharge surfaces are arranged so as to be offset from the convective internal cooling passages, so that the internal cooling takes place precisely where the film cooling is less effective.